Band filter



M. VOS ET AL FebJ2, 1932.

BAND FILTER w11ea Feb. 14, 1929 3 SheetsSheet 1 Feb. 2, 1932. M. vos ETAL 1.843,892

BAND FILTER Filed Feb.*l4, 1929 5 Sheets S heet 2 Feb. 2, 1932. M. vosET AL 3, 9

BAND FILTER Filed Feb. 14,. 1929 3 Sheets-Sheet 3 Patentecl Feb. 2, 1932UNITED STATES PATENT-FOFFICE MAURIIZ vos ND VIIKAI\T Knnn AGUST STERKY,or STOKHOLM, SWEDEN, AS-

smnons T TELEFONAKTIEBLAGE L, M. nRmsso1v, on s rocxnorn, svvnnnn,- A

" BAND FILTER Applicatio filed February 14, 1929, Serial No. 339,946,and in Sweden '1"ebri1ary 27, 1928.

The present invention relates to s0 called band filters which are usedin telephone and high frequencysysterhs to sejoarate oscillations ofdifierent frequencies. or difierent bai1ds 0f frequencies. Especially inconnection 'With multiple telephon or telegraphy,

it often occurs that two or sveral band filters of difierent bands offrequenbies have to be connected in parallel oi in series in relation toeachbther either on the input sideS or on the output sides. In'hithrtoknown simple bahd filters i. e. those consisting ofa single section thecharacteristics of the two sides (i.-e. the input and the outputcharacteristic) are similar so far as they are both falling or bothrising on each side of the band of' frequen oies of the filter. In theformer case the filter is not suitable forbeing connected in parallelbecause the charactefistic of each of the filters has 10W values foroscillations within the band of requenciesof the other filters thedifit'erent filters connected in parallel thus forming short circuitsfor each other. Filters of the latter kind in which the characteriSticis rismg on both sides of the band of frequericies are, on the otherhand, not suitable for being connected in series becausein this caseeach of the filters connected in series forms a very high impedance foroscillations within the band of frequencies of the other filters. Theobject 'of the present invention is to bring about a simple band filter,which may be connected advantageousl both in parallel and in series. Theobject of the invention is further to produce an efiicacious filter of adesign so simple asypossible and to combine in such a simple filter allthose qualities which are desirable in a bandfilter and which otherwisecan be*obtained only by comparatively complicated devices. The inventionWill be more closely de scribed with reference to the attache-d drawingsin which Figure 1 shows an embodiment of a single section filter.Figures 2 and 3 are diagrams showing the variations of the input andoutput characteristics of the filter according to Figure 1, as a unctionof the frequency. Figure 4 shows the connection of the filter accorclingto Figurl to the xnode.side of an electron tube and Figure 5 theconnection of said filter to the grid Side of an electron tube. Figure 6is a diagra n showing the damping as a function of the frequency in thefilteraccording to Figure 1. Figures 7 and 8 show diflzerent connectionsin cascadeor in chain et sections according to Figurel. Figures9 and.l0show modifications ofthe single section filter according to Figure 1.Figures 11 and 12 show connections in series or in parallel respectivelyof filtersaccording to the.inventioh[ The single section filteraccording to Figure lis composed of two induetively coupledinductances:Lf and L2 and a condenser C connectd in parallel with L anda condenser C connected in series with L The input terminals 1 of thefilter are formed by the terminals of theinductance L Which areconnectedto the condenser C The free end of the inductance L and theouter p.0le of the condenser C form the output termirials 2 of thefilter. By suitably selecting the ratio et transformation and thecoupling coefficient 76 and by a corresponding dimensioning ofinductances and capacities the filter may.be adjusted to suit twoimpedancesZ and Z (limes or apparatus) connected to the input 7 andoutput terminals' respectively in;such a manner that oscillations withinthe desired band of frequencies pass the filter withoutreflectionlosses. The impedances Z and Z which may be of difi'erentvalues in relation to each other may be assumed to. have, at leastapproximately, purely realvalues o:t inupedance ithin the band offrequencies and especially for the geometric mean value of thetwojlimiting. frequencieS f f of the filter. Provicled the filteris thusadjusted t0 predetermined values of:Z and Z,at the geometric -meanfrequency betweenf and f and provided the losses in coils and condensersare negligible we obtain accorcling to the general theory of the idealartificial line With two input and two output terminale or the so calledtransducer (see for instance Breisig, Theoretische Telegraphie, 2ndedition, page 401) the following formulas for a suitable dimensioning ofthe filter according to Figure 1.

An arrangement of the filter as described presents the followingadvantages:

A. As can be readily understood from the diagrams in Figures 2 and 3which show the input and output characteristics of the filter drawnaccordingto a symbolical method set forth by Zobel among others (TheBell System Technical Journal for J anuary 1925, page 62) the filter maysuitably be connected in series with other similar filters (having,hoWever, difierent bands) on the input side 11 whereas the output side22 may suitably be connected in parallel.

B. A filter section according to the invention has an impedance both onthe input and the output side Which is practically constant over thegreater portion of the band of frequencies.

C. On account of the transformation within the filter said filter may beconnected without reflection losses to two impedances of differentsizes. The band width is then as can be seen by the third formula notdependent upon the ratio of transformation of the transformer anddetermined solely by the coupling coefficient 7).

D. When connecting the filter section into the anode circuit of anamplifying tube according to Figure 4: the anode may be directly fed bycontinuons Current over the coil L without introducing a special chokecoilin the supply lead and a blocking condenser or other couplingelements,

E. W'hen connecting the filter section into the grid circuit in anaudion tube it is usually desirable to dimension the side of the filterfacing the gridfor a high impedance i. e. to design the filter with ahigh ratio of transformation. The maximum attainable transformationratio is then, however, limited by that the internal capacity of theinductance coil L apparently reduces the value of L In the presentfilter one may, however, as-

sume that both the internal coil capacity and the grid circuit capacityare inclucled in 0 the filter thus functioning independently ofotherwise disturbing internal coil capacity.

F. The filter section according to the inimpedance diagrams representpurely imag- 'inary values of the impedances (pure inductances andcapacitances) whereas the unbroken portion of the impedance diagramsextending between the limiting frequencies f f in each of the figuresrepresents purely real values (ohmic resistances).

In order to render the filter symmetrical the condenser 0 connected inseries with the inductance L (Figure 1) may be divided into twocondensers each having the capacity 2C2 which are connected into circuitaccording to Figure 9. In this way another embodiment of the inventionis realized.

It is also possible to unite a number of filter sections according tothe invention by means of cascade coupling resulting in circuit diagramsaccording to Figures 7 and 8. In such Cases When two coi1densers areconnected into circuit in parallel or in series by means of said cascadecoupling they may, of course, be combined to one condenser having thedouble or the half capacity respectively.

A1so other embodiments of the filter in addition to the above mentionedones are conceivabl such as for instance the substitution of thetransformer L L by its equivalent circuit diagram according to Figure10. Thus:

filters connected in series ofiers thus only' an insignificant impedancefor oscillations within the band of frequencies of the other filters.

The cohnection in parallel of several fil- H to Figure 12 on those sideswhere the series capacities are connected into circuit. The differentfilters ofler thus' according to Figure 3 high impedance values forother oscillations than those lying within each of the appertaining bandof frequ&encies.

We claim: 7

1. A bond filter section consisting of two inductively coupledinductances (L L a 10 capacity (0 connected in parallel with the oneinductance (L and a capacity (C connected in series with the otherinductance (L the inductances and the capacities and the couplingcoefiicient (la) between the in-- ductances being so dimensioned thatsubstantially A and f being the limiting frequencies and Z Z theterminal impedances, one of the two characteristics of the filter thenfalling on both sides of the band of freqnencies and the other risingonboth sides of said baud. 2. A band filter as claimed in claim 1,connected to anapparatus with inner capacity, characterized by that saidinner capacity is includecl in the parallel condenser of the filter. 3.A bond filter as claimed in claim 1, characterized in thzit two orseveral sections having diflerent band of frequencies are connectecl inseries in relation to each other on those sides where theircharacteristics are falling on both sides of the corresponding band offrequencies whereas the opposite sicles are connected to difi'erentimpedances. 4:. A band filter as claimecl in claim 1, characterized inthat two or several sections having diffrent band of requencies areconnected in parallel in relation to each other on those sicles wheretheir charactertistics are rising on both sides of the correspondingband of frequencies whereas the opposite sicles are connected todifl?erent impedances.

In testimony whereof we aifiX our signaturcs.

MAIRITZ VOS. m HKAN KARL AUGUST STERKY.

